EP0067338A2 - Method for the correction of a magnetic field probe - Google Patents

Abstract

In a correction system for a magnetic field probe, the probe is first moved in a circle for calibration such that respective maximum and minimum values of a measured magnetic field vector or a respective component thereof is determined. By means of a sum formation and halving of the respective maximum and minimum value, a respective additive correction value is derived which is added to the respective measured values during navigation with the magnetic field probe. By so doing, a noise vector superimposed on the external magnetic field is eliminated.

Description

The invention relates to a correction method for a magnetic field probe, in which an external magnetic field, in particular the earth's magnetic field, is measured according to size and direction. The invention also relates to a device for carrying out this method.

Magnetic field probes as electronic compasses are already known in various forms. For example, such probes are constructed with two mutually perpendicular coils, the induction of which is influenced by the earth's magnetic field or another external magnetic field. Measuring methods that take advantage of this effect are described, for example, in German patent applications P 29 49 815, P 30 29 532 and P 31 21 234.

When using compasses, in particular in vehicles, for example in ships, airplanes or also in land vehicles, various errors occur which are manifested by a refusal. This rejection arises on the one hand from unwanted magnetic fields in the vehicle or in the probe itself, and on the other hand from an incorrectly horizontal installation of the probe. Another error can result from the type of probe. If, for example, the two components of the magnetic field are measured in the scan with two different coils, errors can result from inequalities of the two coils or in the evaluation electronics.

In order to avoid errors in compasses, it is already known to slowly rotate the vehicle by 360 ° with the built-in compass on a compensation platform. A table is created point by point from which the assignment between the displayed and the actual angle in relation to the north direction is shown. Another possibility of correction is to reduce the refusal with the aid of individually attached compensation magnets. Both measures are complex to implement. In particular, they are difficult to automate or they require a lot of equipment.

The object of the invention is the correction of a magnetic field probe with simple means and without complex devices, such as a compensation platform. This correction should be very precise and lead directly to the output of the correct measurement result, so that, for example, conversion tables and the like are superfluous.

According to the invention, the object is achieved in that the probe moves in a circle before the start of the measurement and, by continuously measuring the magnetic field vector relative to the probe, its maximum and minimum values are determined, that a correction vector is formed from the halved vector sum of these values and later during the current one Measurement is vectorially added to the measured values of the probe.

The invention is based on the knowledge that all disturbing influences due to undesired external magnetic fields can be added to a disturbing vector and that this disturbing vector can be compensated for by the same amount by adding an oppositely directed vector. Since there is a very specific direction in which the field vector of the magnetic field to be measured coincides with the interference vector in the direction, it is in this direction Direction measured a maximum value, while with a rotation of 180 ° compared to this maximum value, the interference vector is subtracted from the field vector and thereby produces a minimum measurement value. By measuring the maximum and minimum values according to the invention and by halving the vector sum, the interference vector can thus be determined and a correspondingly opposite correction vector can be stored, which is then automatically added to each measurement result during the magnetic field measurement.

The implementation of the correction method depends on the type of measurement method used or on the type of probe. If the measurement is carried out in the polar coordinate system -, the correction vector can be obtained by vectorially adding the absolute maximum and minimum values of the magnetic field vector and by halving and inverting the vector sum. In this case, the angle of the correction vector is shifted by 180 ° in relation to the maximum value of the field vector determined during the calibration.

If the measurement is taken in a Cartesian coordinate system v _D T, the maximum and the minimum value are expediently measured separately for the two mutually perpendicular components of the magnetic field vector and then the amount of the minimum value is subtracted from the amount of the maximum value for each component, resulting in halving A separate correction value can be determined for each of the two components. This correction value is then added separately to the measured value of the respective vector component.

As mentioned at the outset, when measuring separate magnetic field components, there is a risk that inequalities in the coils used or in the evaluation electronics can also lead to the compass failing. To Compensation of this error is provided in a development of the invention that the sum of the absolute maximum and the absolute minimum value is formed for the two components of the magnetic field vector and that the quotient of the sum value of the first and the second measured component as a correction factor for Measured values of the second component is used.

If the magnetic field probe is installed in a vehicle, the vehicle is expediently driven in a circle for calibration, the additive correction values and, if appropriate, the correction factor being determined and stored. This calibration run can be carried out at any point, for example in a parking lot. Special turntables, compensation platforms or similar devices are not required.

To carry out the process of the invention, means are advantageously provided, is switchable in which one of Magnetfeldsondenachgeschaltete evaluation circuit with its output via a switching means alternatively to a calibration arrangement and on eineMeßanordnung with downstream display device, further wherein there is provided a correction value memory, having its ^- input at Calibration arrangement and with its output connected to the measuring arrangement. The calibration arrangement can have comparison devices for determining maximum and minimum values, adders connected downstream of the comparison devices and dividers connected downstream thereof, the outputs of which are fed to the correction value memory. Each comparison device expediently has threshold switches fed back via buffers for both positive and negative values, with an absolute value generator and a memory for the maximum and the minimum value being connected downstream of these. An adder is expediently provided in the measuring arrangement, in which additive correction values are added to the respective measured values from the correction value memory. In addition, a multiplier can be provided in the measuring arrangement, in which at least the measured values of a magnetic field component are multiplied by a correction factor supplied from the correction value memory. A divider, an angular function converter and a display device can be connected downstream of the adder or multiplier in a manner known per se. For a navigation device in a vehicle, the output of the angular function converter is expediently fed to a display device together with the output of a displacement sensor via an integrator. The direction and the distance from a starting point or to a destination in the vehicle can thus be displayed by means of a suitable display device.

• Figur 1 eine Prinzipdarstellung eines zu messenden Magnetfeldvektors mit Störvektor in einem Polarkoordinatensystem,
• Figur 2 die prinzipielle Gewinnung von Korrekturwerten in einem cartesischen Koordina-tensystem,
• Figur 3 die Darstellung einer Meßwertverzerrung durch unterschiedliche Spulen für zwei Magnetfeldkomponeten in einem Polarkoordinatensystem,
• Figur 4 den Verlauf der Einzelmeßwerte für zwei Magnetfeldkomponenten bei einer Kreisfahrt,
• Figur 5 eine Prinzipschaltung zur Gewinnung von Korrekturwerten,
• Figur 6 eine Prinzipschaltung für die Meßanordnung zur Gewinnung korrigierter Meßwerte,
• Figur 7 eine detailliertere Prinzipschaltung für eine Vergleichseinrichtung aus Figur 5.

The invention is explained in more detail below using exemplary embodiments with reference to the drawing. It shows

• FIG. 1 shows a basic illustration of a magnetic field vector to be measured with an interference vector in a polar coordinate system,
• FIG. 2 shows the principle of obtaining correction values in a Cartesian coordinate system,
• FIG. 3 shows a measurement value distortion by different coils for two magnetic field components in a polar coordinate system,
• FIG. 4 shows the course of the individual measured values for two magnetic field components during a circular drive,
• FIG. 5 shows a basic circuit for obtaining correction values,
• FIG. 6 shows a basic circuit for the measuring arrangement for obtaining corrected measured values,
• FIG. 7 shows a more detailed basic circuit for a comparison device from FIG. 5.

FIG. 1 schematically shows the magnetic field measurement with and without interference in a polar coordinate system. At the center M is the magnetic probe or the vehicle carrying the probe. The probe is used to measure a vector describing the external magnetic field, which in each case indicates the amount and direction of the external field lines with respect to a defined central axis of the probe or of the vehicle. The vector v _{1 describes} the external magnetic field (earth's magnetic field). If there were no interferences, the magnitude of this vector would always be the same for a circular drive. The tip of the vector would move around the center M on the circle K1.

However, if an additional interference vector v _{2 is} present, this interference vector is added to the magnetic field vector v _{1 '} so that the magnetic field vector v ₁ apparently rotates around a center point M'. Therefore is measured or calculated when the vehicle Kreisfaihrt a vector v _3, the v from the vectorial addition of the fixed interference vector ₂ and in accordance with the vehicle motion variable magnetic field vector v ₁ is obtained. This calculated vector v ₃ differs from the actual field vector v ₁ not only in magnitude, but also in angle a ₃ compared to the actual angle α ₁ .

Der Winkel des Störvektors entspricht dem Winkel von v_max, da in diesem Moment

The measured vector v ₃ then reaches its maximum value v _max when the angle of the earth's magnetic field a ₁ coincides with the angle of the interference vector a ₂ , and on the other hand it reaches the minimum when rotated through 180 ° when the magnetic field vector v ₁ and the interference vector v ₂ overlap exactly opposite. By measuring these extreme values v _max and v _min , the interference vector can be determined, namely according to the amount:

The angle of the interference vector corresponds to the angle of v _max because at this moment

A correction vector must therefore be exactly the opposite of the interference vector v ₂ and correspond to it in terms of amount.

und

If the magnetic field with its mutually perpendicular components f ₁ and f _{2 is} measured in a Cartesian coordinate system according to FIG. 2, a correction value can also be determined separately for the two components. As already mentioned with reference to FIG. 1, the true magnetic field vector v ₁ would have to remain the same in the case of a circular movement of the probe and execute a circle K1 around the center point M. Due to the interference vector v ₂ , however, the center of the measured circle K1 'is shifted from M to M'. An apparent magnetic field vector v ₃ with its components f and f ^' ₂ is therefore measured. In order to determine the true magnetic field components f ₁ and f ₂ , the correction values k ₁ and k ₂ which correspond to the x and y components of the interference vector v ₂ must be subtracted from the measured values. To determine these correction values, one measures the maximum and minimum values of the components on the x and y axes, namely f _1max and f _1min or -f _2max and f _2min . The correction values according to the relationship are then obtained from this

and

The influence of the interference vector v ₂ can thus be eliminated by subtracting these correction values k ₁ and k ₂ from the measured values f ' ₁ and f' ₂ . There are no interferers flows, the amounts f _1max and f _1min and ^w . f _2max and f _2min are the same, so that the correction value becomes zero.

Another possibility of error in the field measurement can be that the two field components f ₁ and f _{2 are determined} with different coils or with different evaluation circuits. This can result in a distortion of the measured values, as shown in FIG. 3. The field vector v ₁ with the components f ₁ and f _{2 should} lie on a circle K when the probe is rotated. The angle α can be calculated from the two components and thus the direction of the vector v ₁ can be determined. However, if f ₁ and f ₂ are measured with mutually different coils or evaluation circuits, a scale shift occurs, as a result of which the circle K degenerates into an ellipse K '. So f " ₁ and f ₂ are related to each other, which results in a distorted angle α '.

A correction of this distortion is based on the knowledge that if the external field remains the same, the amplitude values for both components must be of the same size, and that both f ₁ and f ₂ must go through the same amplitude swing. The acquisition of a correction factor for eliminating the measurement distortion is described with reference to FIG. 4. The behavior of the individual measured values f ₁ and f ₂ as a function of the angle of rotation a of the probe is shown. The dashed value is the measured value f ' ₁ or f' ₂ . These measured values are shifted with respect to the zero line, specifically because of an interference vector which is eliminated by the correction values k ₁ and k ₂ . This results in the corrected measured values f " ₁ and f ₂ .

und

nicht verändert. Der Gesamthub der Meßwerte für beide Komponenten muß jedoch gleich groß sein. Daraus läßt sich ein Korrekturfaktor m ermitteln:

This correction compensates for the course of the individual measured values compared to the zero line, but the respective total stroke

and

not changed. However, the total stroke of the measured values for both components must be the same. A correction factor m can be determined from this:

The distortion can be compensated for by multiplying the measured values f ₂ by m.

A basic circuit for carrying out the correction described will now be shown with reference to FIGS. 5 to 7. A magnetic probe MS with a subsequent evaluation circuit AWS is required. The probe has, for example, two mutually perpendicular coils, the two components of the external magnetic field being obtained by a known method, for example according to German patent application P 29 49 815, P 30 29 532 or P 31 21 234. These measured values f ' ₁ and fl2 are present at the output of the evaluation circuit AWS. With the switch US1, these outputs can be switched either to a calibration arrangement according to FIG. 5 or to a measuring circuit according to FIG. 6. For calibration purposes, the switch US1 connects the output of the evaluation circuit AWS to two comparison devices VG1 and VG2, in each of which the maximum and minimum measured values for f ' ₁ and f' _{2 are} determined and stored. A more detailed illustration of such a comparison device is shown in FIG.

To calibrate the system, the probe or a vehicle with a built-in probe is driven in a full circle. After this calibration run, the comparison devices VG1 and VG2 each have the absolute values of f _1max , f _1min as well as f _2max and f _2min . The switch US2 then switches the outputs of the comparison devices VG1 and VG2 to the subsequent adders ADD1 to ADD4, in which the stored measured values are linked. The difference between the amounts of f _1max and f _1min or f _2max and f _{2min is} formed in the two adders ADD2 and ADD4. In the following dividers DIV1 and DIV2, these ascertained differences are each divided by two and thus result in the additive correction values k ₁ and k ₂ , which are entered into a correction value memory SPK and kept ready for use in navigation.

In the adders ADD1 and ADD3 the sum of the absolute values of f _lmax and f _1min as well as of f _2max and f _{2min is} formed. In the divider DIV3, the quotient is formed from the two sums of the adders ADD1 and ADD3, which results in the correction factor m. This correction factor m is also stored in the memory SPK. After the calibration, both additive and multiplicative correction values are available in the correction value memory SPK for the actual navigation of the magnetic probe.

The switch US1 is brought into the position according to FIG. 6 for navigation. As a result, the output of the evaluation circuit AWS is switched to the adder ADD5, so that the measured values f ₁ 'and f' ₂ are continuously fed into this adder while the vehicle is traveling. In addition, the adder ADD5 receives the additive correction values k ₁ and k ₂ from the correction value memory SPK. These are then added to or subtracted from the respective measured values f ' ₁ or f' ₂ . A multiplier MUL is connected downstream of the adder ADD5 and also receives the correction factor m from the correction value memory SPK. At least one of the - possibly corrected in the ADD5 adder - Measured values are multiplied by the factor m in the multiplier MUL. If necessary, both measured values can additionally be multiplied by a further factor in order to achieve a certain scale for the further calculation.

The corrected measured values for the two magnetic field components are thus present at the output of the multiplier MUL. The direction of the magnetic field can now be determined from this in a simple manner, since one measured value is proportional to sine a and the other measured value is proportional to cosine a. From these values, the tangent a can be calculated in the following divider DIV4 and the angle a in the downstream angle function converter WU. This angle a can either be displayed directly in the display device AN1 or processed further in a downstream integrator INT. For example, the distance s traveled can be fed to the integrator INT from a distance sensor WG. In this case, both the angle and the distance of the route covered or, if appropriate, the route to a desired destination can be displayed in a display device AN2. For example, the arrow R could indicate the direction to the destination and the seven-segment display S the straight route to this destination.

7 shows the example of VG1 obtaining the maximum and minimum values in the calibration arrangement according to Figure 5. the switch US1 each measured value of f _'1 in the two threshold is input SWS1 and SWS2. The first threshold switch SWS1 switches through at any value that is greater than the previous value of f ' ₁ , while the threshold switch SWS2 switches through at any value that is less than the previous value of f' ₁ . Via a buffer ZSP1 or ZSP2 Each new value after switching on the threshold switch SWS1 or SWS2 is entered in a threshold value memory SPS1 or SPS2. The content of the threshold value memory SPS1 or SPS2 forms the new threshold for the associated threshold value switch SWS1 or SWS2. After a complete circular travel of the vehicle with the probe MS, the maximum value f ' _{1 is present} in the buffer store ZSP1 and the minimum value of f' ₁ in the buffer store ZSP2. From this, the magnitude of f _1max and f _{1min is} formed in the absolute value images ABS1 and ABS2 and stored in the subsequent maximum value memory SPM1 and the minimum value memory SPM2. From there, these absolute amounts can be processed according to FIG. 5.

Claims (12)

1. Correction method for a magnetic field probe, in each of which an external magnetic field, in particular the earth's magnetic field, is measured for size and direction, characterized in that the probe (MS) moves in a circle before the start of the measurement and thereby by ongoing measurement of the magnetic field vector ( v ₃ ) compared to the probe (MS), its maximum and minimum values (v _max , v _min ; f _1max , f _2max , f _1min , ^f 2min) determined that a correction vector was formed from the halved vector sum of these values and later during the current one Measurement is vectorially added to the measured values of the probe. 2. The method according to claim 1,
characterized ,
that a polar coordinate system is used for the measurement, the correction vector being obtained by vectorially adding the absolute maximum and minimum values (v _max , v _min ) of the magnetic field vector (v ₁ ) and by halving and inverting the vector sum in terms of amount. 3. The method according to claim 1,
characterized ,
that a Cartesian coordinate system (x, y) is used for the measurement, whereby for the two mutually perpendicular components (f ₁ , f ₂ ) of the magnetic field _vector (v ₁ ) the maximum and minimum values (f _1max , f _1min , f _{2max ^'f 2m} i _n) is measured, and d is the amount _ate for each component the minimum value of the magnitude of the maximum value and subtracts a separate additive correction value is determined therefrom by halving and inversion, respectively. 4. The method according to claim 3,
characterized ,
that the sum of the absolute maximum and minimum value is formed for the two components (f ₁ , f ₂ ) of the magnetic field vector and that the quotient of the sum of the first and second components is used as a correction factor (m) for the measured values of the second component (f ₂ ) is used. 5. The method according to any one of claims 1 to 4,
characterized ,
that a vehicle carrying the magnetic field probe (MS) is driven once in a circle to obtain the correction values. 6. Device for performing the method according to claim 1,
characterized,
that the magnetic field probe (MS) is followed by a known evaluation circuit (AWS), the output of which can be switched via a switching device (US1) either to a calibration arrangement or to a measuring arrangement with a downstream display device (AN1, AN2), and that a correction value memory ( SPK) is provided, which is connected with its input to the calibration arrangement and with its output to the measuring arrangement. 7. Device according to claim 6,
characterized ,
that the calibration arrangement has comparison devices (VG1, VG2) for determining maximum and minimum values, adders (ADD1 to ADD4) connected downstream of the comparison devices (VG1, VG2) and dividers (DIV1 to DIV3) connected downstream thereof, the outputs of which are fed to the correction value memory (SPK) are. 8. Device according to claim 7,
characterized,
that each comparative direction (VG1, VG2) via buffer (ZSP1, ZSP2) feedback threshold switches (SWS1, SWS2) has both positive and negative values, and that these each have an absolute value generator (ABS1, ABS2) and a memory (SPM1 , SPM2) for the maximum or minimum value. 9. Device according to one of claims 6 to 8,
characterized ,
that an adder (ADD5) is provided in the measuring arrangement, in which additive correction values (k ₁ , k ₂ ) are added to the respective measured values (f ₁ , f ₂ ) from the correction value memory (SPK). 10. Device according to one of claims 6 to 9,
characterized ,
that a multiplier (MUL) is provided in the measuring arrangement, in which at least the measured values of a magnetic field component (f ₂ ) are multiplied by a correction factor (m) supplied from the correction value memory (SPK). 11. Device according to one of claims 9 or 10,
characterized ,
that the adder (ADD5) or the multiplier (MUL) is followed by a divider (DIV4), an angular function converter (WU) and a display device (AN1, AN2). 1.2. Device according to claim 11,
characterized ,
that the output of the angular function converter (WU) is fed together with the output of a displacement sensor (WG) via an integrator (INT) to a display device (AN2).

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